science

Isolation amplifier

Isolation amplifier is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Isolation amplifier rather than just read about it. In short: Isolation amplifiers are a form of differential amplifier that allow measurement of small signals in the presence of a high common mode voltage by providing electrical isolation and an electrical safety barrier. They protect data acquisition components from common mode voltages, which are potential differences between instrument ground and signal ground.

Key takeaways

Isolation amplifier belongs to science; place it in that map before memorising details.
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Connect Isolation amplifier to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Isolation amplifier from memory before moving on to harder problems.

Reference excerpt

Isolation amplifiers are a form of differential amplifier that allow measurement of small signals in the presence of a high common mode voltage by providing electrical isolation and an electrical safety barrier. They protect data acquisition components from common mode voltages, which are potential differences between instrument ground and signal ground. Instruments that are applied in the presence of a common mode voltage without an isolation barrier allow ground currents to circulate, leading in the best case to a noisy representation of the signal under investigation. In the worst case, assuming that the magnitude of common mode voltage or current is sufficient, instrument destruction is likely. Isolation amplifiers are used in medical instruments to ensure isolation of a patient from power supply leakage current. Amplifiers with an isolation barrier allow the front-end of the amplifier to float with respect to common mode voltage to the limit of the barrier's breakdown voltage, which is often 1,000 volts or more. This action protects the amplifier and the instrument connected to it, while still allowing a reasonably accurate measurement. These amplifiers are also used for amplifying low-level signals in multi-channel applications. They can also eliminate measurement errors caused by ground loops. Amplifiers with internal transformers eliminate external isolated power supply. They are usually used as analogue interfaces between systems with separated grounds. Isolation amplifiers may include isolated power supplies for both the input and output stages, or may use external power supplies on each isolated portion.

Concepts

Signal source components

All signal sources are a composite of two major components. The normal mode component (VNM) represents the signal of interest and is the voltage that is applied directly across the inputs of the amplifier. The common mode component (VCM) represents the difference in potential between the low side of the normal mode component and the ground of the amplifier that is used to measure the signal of interest (the normal mode voltage). In many measurement situations the common mode component is irrelevantly low, but rarely zero. Common mode components of only a few millivolts are frequently encountered and largely and successfully ignored, especially when the normal mode component is orders of magnitude larger. The first indicator that common mode voltage magnitude is competing with the normal mode component is a noisy reproduction of the latter at the amplifier's output. Such a situation does not usually define the need for an isolation amplifier, but rather a differential amplifier. Since the common mode component appears simultaneously and in phase on both amplifier inputs, the differential amplifier, within the limits of the amplifier's design, can reject it. However, if the sum of the normal mode and common mode voltages exceeds either the differential amplifier's common mode range, or maximum range without damage then the need for an isolation amplifier is firmly established.

Operating principles Isolation amplifiers are commercially available as hybrid integrated circuits made by several manufacturers. There are three methods of providing isolation. A transformer-isolated amplifier relies on transformer coupling of a high-frequency carrier signal between input and output. Some models also include a transformer-isolated power supply, that may also be used to power external signal processing devices on the isolated side of the system. The bandwidth available depends on the model and may range from 2 to 20 kHz. The isolation amplifier contains a voltage-to-frequency converter connected through a transformer to a frequency-to-voltage converter. The isolation between input and output is provided by the insulation on the transformer windings. An optically isolated amplifier modulates current through an LED optocoupler. The linearity is improved by using a second optocoupler within a feedback loop. Some devices provide up to 60 kHz bandwidth. Galvanic isolation is provided by the conversion of electric current to photonic flux through the space between the LED and the detector, regardless of the intervening medium. A third strategy is to use small capacitors to couple a modulated high-frequency carrier; the capacitors can stand off large DC or power frequency AC voltages but provide coupling for the much higher frequency carrier signal. Some models on this principle can stand off 3.5 kilovolts and provide up to 70 kHz bandwidth.

Isolation amplifier usage Isolation amplifiers are used to allow measurement of small signals in the presence of a high common mode voltage. The capacity of an isolation amplifier is a function of two key isolation amplifier specifications:

The amplifier’s isolation breakdown voltage, which defines the absolute maximum common mode voltage that it will tolerate without damage. Specifications of 1,000 volts and more are common. The amplifier’s common mode rejection ratio (CMRR). The CMRR specification defines the degree to which the common mode voltage will disrupt the normal mode component measurement, and therefore affect measurement accuracy. The frequency of the common mode voltage can adversely affect performance. Higher frequency common mode voltages create difficulty for many isolation amplifiers due to the parasitic capacitance of the isolation barrier. This capacitance appears as a low impedance to higher frequency signals, and allows the common mode voltage to essentially blow past the barrier and interfere with measurements, or even damage the amplifier. However, most common mode voltages are a composite of line voltages, so frequencies generally remain in the 50 to 60 Hz region with some harmonic content, well within the rejection range of most isolation amplifiers.

Differential amplifiers A non-isolated differential amplifier does not provide isolation between input and output circuits. They share a power supply and a DC path can exist between input and output. A non-isolated differential amplifier can only withstand common-mode voltages up to the power supply voltage. Similar to the instrumentation amplifier, isolation amplifiers have fixed differential gain over a wide range of frequencies, high input impedance and low output impedance.

Amplifier selection guidelines Instrumentation amplifiers can be classified into four broad categories, organized from least to most costly:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Isolation amplifier

Start with the simplest possible case. Write down what Isolation amplifier claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Isolation amplifier before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Isolation amplifier ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Isolation amplifier

In research: Isolation amplifier appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Isolation amplifier in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Isolation amplifier is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic amplifiers, Electronic design, so understanding it makes those chapters shorter.
In everyday life: Look for Isolation amplifier outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Isolation amplifier in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Isolation amplifier means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Isolation amplifier out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Isolation amplifier in simple terms?

Why does Isolation amplifier matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Isolation amplifier?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Isolation amplifier.